Speeding up of a rotating polygonal mirror, thereby speeding up of photo scanning is needed to realize a high-speed laser printer. Furthermore, in an optical modulation type laser printer performing an optical modulation in response to data to be printed, a high-speed modulation is needed to a high-speed laser printer.
A laser printer using a multi-beam is an effective means to a high-speed laser printer because it can reduce a rotational speed of a rotating polygonal mirror and an optical modulation speed by the number of beams in the multi-beam. For the multi-beam type laser printer to realize more high-speed printing, it is necessary to increase the number of beams in the multi-beam.
FIG. 3 shows a first example of a laser printer using a plurality of semiconductor lasers, particularly an optical system thereof. FIG. 7 shows a sectional view of an optical fiber array in a multi-beam generating section.
In FIG. 3, a semiconductor laser module 1 comprises a plurality of semiconductor lasers 2 and optical fibers 3 for leading the respective laser beam from semiconductor lasers 2. A plurality of optical fibers 3 are aligned in a row at an optical fiber array unit 4.
As shown in FIG. 7, the optical fiber array unit 4 is configured such that the optical fibers 3 whose sheath has been removed, are held in V-shaped grooves 11 formed by applying anisotropic etching to Si crystal, while being pressed against the V-shaped grooves 11 with a glass plate 12 and being fixed with an adhesive 13.
Each optical fiber 3 comprises a clad portion 31 and a core portion 32, and light propagates via the core portion 32. An outer diameter of the clad portion is generally about 125 μm and hence the distance between adjacent core portions is 125 μm even when the optical fibers are aligned in nearness layout to each other.
In the case of an optical system shown in FIG. 3 wherein a multi-beam having the above beam distance is used, since each beam distance in the multi-beam is comparatively large, if the number of beams in the multi-beam increases, the beams at both sides of the optical fiber layout largely deviate from an optical axis of the optical system. Accordingly, deterioration of aberration characteristics of optical system components may be caused.
For the reason, when using the lenses 6 and 8 in FIG. 3 for the multi-beam laser printer, it becomes necessary to use lenses with a high degree of accuracy to maintain good aberration characteristics even when the beams deviate largely from the optical axis in comparison with the case of using only one beam.
Consequently, it is impossible to increase the number of beams in a multi-beam and high speed printing is limited.
In a second example of an optical system in a laser printer using a multi-beam, a method of using a semiconductor laser array as a multi-beam generating device as shown in FIG. 5 is also possible. However, pitch narrowing is limited due to the problems such as thermal crosstalk, droop, and the like, and the same problem as the first example occurs.
As a method for solving such problems, an optical recording device shown in FIG. 4 is proposed as follows. In the optical recording device, an optical waveguide device 5 is connected to an output end of an optical fiber array 4 in which optical fibers are aligned in nearness layout to each other; and a narrow-pitched multi-beam emitted from the optical waveguide device is scanned on a photosensitive material 9 in batch. This method is disclosed in Japanese Patent Laid-Open Publication No. 11-271652.
Additionally, another recording device shown in FIG. 6 is proposed as follows. In the optical recording device, an optical waveguide device 5 is connected to an output end of a semiconductor laser array 36. Thus, using a narrow-pitched multi-beam emitted from the optical waveguide device 5 is also possible.
In the above-mentioned prior arts using the optical waveguide device, when pitch conversion of the multi-beam is applied by using a downsized optical waveguide device under a trend where the number of beams in a multi-beam increases, it is necessary to increase a curvature of a waveguide and thereby loss in the optical waveguide device may be caused.
In order to realize a curved waveguide of low loss even with a small curvature radius, it is necessary to increase a difference in refractive index between a core layer and a clad layer and narrow a waveguide width.
However, merely increasing the difference in refractive index between the core layer and the clad layer and narrowing the waveguide width, the loss of connection between the optical waveguide device and optical fibers or a semiconductor laser array undesirably increases. Accordingly, even if bend loss in an optical waveguide device can be reduced, that may not result in the improvement of the optical utilization efficiency of the whole multi-beam light source section in some cases.
A method for solving such problems is described in Paper No. C-3-148, Electronics 1, the Proceedings of the General Convention of the Institute of Electronics, Information and Communication Engineers published in 1998, or in Japanese Patent Laid-Open Publication No. 2000-66048.
The above two documents disclose the following optical waveguide device. The optical waveguide device is configured such that: the difference in refractive index between a waveguide and an over-clad layer is increased only in a curved area of the optical waveguide device to narrow a waveguide width; on the other hand, the difference in refractive index between the waveguide and the over-clad layer are reduce at the optical joint with optical fibers and the like to widen the waveguide width.
However, when using violet semiconductor lasers having a wavelength of 380 to 460 nm in a plurality of single mode optical fibers for leading a multi-beam to an optical waveguide device, it is necessary to increase an allowance of axial deviation at the time when a semiconductor laser beam enters an optical fiber core portion at a semiconductor laser module 1. For the reason, it is needed to reduce the difference in refractive index between a core and a clad, and form a structure in the vicinity of the cutoff of a TEM01 or TEM10 mode where the core diameter is increased.
Additionally, in order to increase the allowance of axial deviation between an optical fiber and the optical fiber joint of the optical waveguide device, it is needed to reduce the difference in refractive index therebetween and increase the core diameter.
Further, in the case of connecting a semiconductor laser array to an optical waveguide device too, in order to increase the allowance of their axial deviation, it is needed to connect a semiconductor laser array having a large outgoing beam diameter like a surface-emitting semiconductor laser array to an optical waveguide device having a large core diameter.
Furthermore, when reducing the difference in refractive index and increasing the core diameter also at the emitting section of an optical waveguide device, it is possible to generate a small pitch multi-beam in rows of beams in relation to a beam diameter in the multi-beam, namely to generate a multi-beam where beams are aligned in a very dense row.
As a result, the influence of aberration in an optical system is suppressed even in the case of a large number of beams and high quality optical recording can be obtained at a high speed.
However, when a multi-beam is subjected to pitch conversion and narrow-pitched in the state of a small refractive index difference and an increased core diameter, a bend loss increases in a curved area in the optical waveguide device.
Further, as shown in FIG. 12, when a waveguide width 19 is narrowed in the curved area in the optical waveguide device and the difference in refractive index between a waveguide 26 and an over-clad layer 27 is increased, a multimode is formed in the vertical direction since a waveguide thickness 20 is not changed and it becomes necessary to convert the beams into a single mode again at an emitting section and thereby loss increases.
An object of the present invention is, even when using violet semiconductor lasers as the light sources, to provide a multi-beam generating device capable of improving optical utilization efficiency at a multi-beam generating section, and to provide an optical recording device using a multi-beam emitted from the multi-beam generating device. The improvement of optical utilization efficiency at a multi-beam generating section is realized by reducing bend loss in an optical waveguide device while the coupling efficiency between optical fibers and the optical waveguide device or between a semiconductor laser array and the optical waveguide device is maintained at a high level.
Further, another object of the present invention is, even when using a large number of beams, to provide a multi-beam generating device to emit a multi-beam from an optical waveguide device that is hardly susceptible to the aberration in an optical system, and to provide an optical recording device realizing high quality optical recording at a high speed using a multi-beam emitted from the multi-beam generating device. The optical waveguide device hardly susceptible to the aberration, is realized by generating a multi-beam having a small pitch multi-beam in relation to the beam diameter in the multi-beam, namely a multi-beam where beams are aligned in a very dense state.